Charged particle beams have a myriad of uses in science and industry. In many applications, great effort is made to maximize the current and/or power in a beam. In the semiconductor industry, for example, ion implanters utilize ion beams to dope semiconducting substrates. Such implanters typically cost several million dollars each.
Given this high cost, it is important that such machines are able to process as many substrates (e.g. silicon wafers) as possible per unit of time, so that the capital cost of the implanter can be amortized over a large number of wafers. The processing rate of an implanter is set by the beam current because each substrate requires a specific fluence. Higher beam currents deliver the same fluence in less time. Thus, higher beam currents result in faster processing per wafer and thereby less cost per wafer.
As another example, charged particle beams are also used in the medical field, where energetic proton beams are routinely used to kill tumors in vivo. Such machines accelerate protons (i.e. hydrogen ions) to high energy and then direct the beam to the specific area of a tumor within a patient. Controlling the proton energy allows the physician to control the precise depth where most of the energy of the proton beam will be deposited. By carefully focusing and sweeping the beam while at the same time regulating its energy, a physician is able to precisely deliver enough localized energy to destroy the tumor while minimizing the damage to adjacent healthy tissue. As with semiconductor ion implantation, a specific fluence (aka “dose”) of protons must be delivered to the tumor, often at multiple energies. Increasing beam current has the potential to increase patient throughput and thereby cost per patient, while at the same time reducing patient discomfort during the procedure.
Those skilled in the art will recognize that there are many other uses of charged particle beams including, for example, materials analysis, materials processing, neutron/radiation generation, nuclear research, and mass spectrometry, to name a few.
As will be discussed in more detail herein below, in one aspect, the invention relates to the use of electrostatic elements or combinations of electrostatic and magnetic elements to confine charged particles in stable recirculating, trapped orbits. Charged particle traps have existed in certain forms for some time. For example, Penning traps use a combination of magnetic and electrostatic fields to confine charged particles of one particular polarity, and the Paul trap uses alternating electric fields for confining charged particles. Purely electrostatic traps have also been developed, as is evidenced by the work at the Weizmann Institute in Israel [H. B. Pedersen et al, Physical Review A, V65, p042703 (2001)] and Stockholm University in Sweden [H. T. Schmidt, Nuclear Instruments and Methods in Physics Research B 173 p523 (2001)]. In essence, these linear traps confine a population of charged particles (typically ions) on stable recirculating orbits within an effective potential well. Ions within a range of kinetic energies bounce between electrostatic mirrors, and focusing elements ensure bounded orbits. In this manner, such devices are reminiscent of stable laser cavities as found in photon optical systems. Typically, the optical properties of these electrostatic traps are such that only particles within a specific and narrow range of energies possess stable orbits. These electrostatic trapping techniques have been developed mainly for use in time of flight mass spectrometry. Particles with different masses but identical kinetic energies will have different orbital periods. Such periods can be measured very accurately, thereby allowing a very accurate measurement of particle mass. Charged particle traps typically store only a limited number of charged particles because, as space charge accumulates, their own Coulomb repulsion distorts the fields of the trap, thereby allowing particles to escape the trap. It will be recognized, by those skilled in the art, that charged particle traps have numerous applications besides mass spectrometry, and that increasing the amount of charge and the range of kinetic energies that can be stored in such traps is of utility.
Also as will be discussed in more detail herein below, in certain embodiments, the invention is directed to improvements to the class of fusion devices based on inertial electrostatic confinement (“IEC”). At present, IEC fusion devices are sold commercially as neutron sources and are also being developed for short-lived isotope production. Neutron sources are used commercially for materials research, detection of fissile materials in shipping containers, and measurement of oil bearing strata about the location of an oil well (known as “well logging”). Certain embodiments of the present invention are directed to greatly increasing the total current impinging upon the core of an IEC device, which increases the useful output of such devices.
Early work by researchers such as Hirsch [R. Hirsch, Journal of Applied Physics 38, p 4522 (1967)], Hu [K. M. Hu & E. H. Klevans, Physics of Fluids, 17, p227 (1974)], Swanson and Verdeyen [Swanson, Ph.D. Thesis, U. of Illinois (1975)], and more recent results by Matsuura [H. Matsuura, K. Funakoshi and Y. Nakao, Nuclear Fusion 43, p989 (2003)] and Meyer [R. M. Meyer, IEEE Transactions on Plasma Science 35, p354 (2007)], investigates virtual electrode formation in spherical IEC devices, and a “multiple well” hypothesis has been proposed. The density profile of such a multiple well is illustrated in FIG. 1. The existence of “Poissors” has been somewhat conjectural, but of great interest because current and density levels in beam-fusion devices might theoretically be raised to useful fusion power producing levels. The general consensus on Poissors is that they might form spontaneously under certain conditions. In all the IEC devices and concepts of the past 40 years, only ions (IXL) or only electrons (EXL, Polywell) are accelerated by potentials on real electrodes, set by one power supply, but never both ions and electrons. Particles of the opposite polarity from the main injection species are envisioned to be confined and/or accelerated via the space charge of the injected species.
Preferred embodiments of the present invention, described in detail herein below, feature the focusing of multiple, ambipolar charged particle beams. There have been some applications where electrons and ions are sent through a common optical element in order to achieve a focused, largely neutral beam, but there are significant differences between these earlier approaches and the embodiments of the present invention.
Beam neutrality is important to prevent the electrostatic charging of various targets, and to prevent beam spreading due to space charge effects. One example is found in beam microscope technologies. The intent of this approach is to merge an electron beam and an ion beam at different energies and then to pass the merged beam through a final lens system that focuses each beam together by way of their difference in energies [H. Liebl, International Journal of Mass Spectrometry and Ion Physics 6, p4-01 (1971)]. Such a system is illustrated in FIG. 2. Another application involves micro-machining using focused and neutralized ion beams [P. W. Ho de Jager et al, Microelectronic Engineering 30, p4-27 (1996)], where an electron beam and an ion beam are merged to form a neutralized beam devoid of net charge build-up. The ‘ambipolar’ lenses used in such systems can be of magnetic or electrostatic nature or a combination of both [J. R. A. Cleaver & H. Ahmed, J. Vac. Sci. Technol. B 3, 144 (1985)].
A series of focusing and defocusing elements in a beam line can have a net focusing effect on both negative and positive charged particles. Such an arrangement can be achieved with magnetic or electrostatic multipoles, or by using a collection of alternating Einzel lenses or a series of electrostatic immersion lenses. The principle is referred to “alternating-gradient” or “strong focusing” and is used in many accelerators today. It is important to note that the focusing takes place regardless of the sign of the charge of the particles, so ions and electrons can both experience focusing along a common beam line.
Ambipolar beam implementations have also been used to increase the current and power carrying capabilities of charged particle beams. The current that may be extracted and transmitted in a beam optical system is limited by space charge considerations, the so-called “Child-Langmuir” limit (alternatively referred to herein as the “CL limit”). This is the limiting current at which a given stream of charged particles may flow in a given external electrostatic field before the space charge of the particles themselves negates the external field and prevents additional current. In addition, secondary effects, such as instabilities, often limit practically attainable beam currents substantially below the Child-Langmuir limit. The use of beams of dual polarity allows current to be increased far beyond what is possible for single polarity ion or electron systems, due to the neutralization of electric charge.